1. Field of the Invention
The present invention relates to magnetic transducers. It is applicable in particular for the reading and writing of data present on a magnetic record carrier, such as rigid or flexible magnetic discs and tapes.
2. Description of the Prior Art
It is known that in order to record data on a magnetic record carrier, at least one modification of one of its magnetic properties is produced on (or in) this carrier at a plurality of perfectly defined points of the carrier, which is manifested by at least one variation of the physical quantity characterizing the said property. Reading of data is performed by detecting these changes and by converting the aforesaid change in physical quantity into a variation of another physical quantity, which is most frequently a variation of the voltage or current of an electrical signal.
For a clear comprehension of the object of the present invention, it may be advantageous to recall a few principles regarding magnetism. First, to magnetize a magnetic material within which the magnetic induction is weak, the material is first exposed to a positive magnetic field H.sub.s whose strength is sufficient to saturate the material, meaning that the magnetic induction in the material reaches a limiting value B.sub.s. The external magnetic field is then removed. A magnetic induction greater than zero, referred to as "remanent induction" B.sub.r and characteristic of the material, then remains in the material. The ratio B/H between the induction and the magnetic field when the field H is caused to change towards zero, is referred to as the "initial magnetic permeability of the material", and this can be generally illustrated on an initial magnetization curve.
An anisotrophic magnetic material has two preferential directions of magnetization at right angles to each other. One of these is referred to as the "direction of easy magnetization". The other is referred to as "direction of difficult magnetization". The initial permeability of the material in the direction of difficult magnetization is considerably greater than the initial permeability of the material in the direction of easy magnetization. The magnetic property utilized to record data on a magnetic carrier is defined, for example, either by the absence or presence of a remanent magnetic induction or by an optional variation of the magnetization.
Two forms of carriers, discs and tapes, are frequently employed in connection with magnetic recording and reading. Magnetic discs carry data on circular concentric recording tracks which have a radial width not exceeding a few hundreths of a millimeter and commonly cover the greater part of both their surfaces. Magnetic tapes carry data on tracks parallel to the tape length.
The means most frequently employed which enable data either to be recorded on carriers such as discs or tapes, or to be read from carriers, or finally to effect the one or the other of these functions, are referred to as "magnetic transducers". As a rule, one or more transducers is or are associated with a record carrier, and the carrier travels or is driven past the transducer. To simplify matters, it will be assumed in the remainder of the description that only one transducer is associated with one carrier; however, it will be apparent to those skilled in the art that a multi-track carrier may have a plurality of transducers associated therewith.
A magnetic transducer comprises a magnetic circuit having pole pieces around which is disposed a winding, and which also comprises a gap. This gap is situated at a very small distance from the carrier surface, ranging from between zero to a few tens of microns. The winding comprises electric input and/or output wires. In order to record data on the carrier associated with the transducer, the winding is supplied with an electric current of which the direction or the period of traversal in said winding may be varied. The carrier is thus exposed to a magnetic field intensity varying in direction, generated by the transducer in direct proximity to its gap, (between zero and a few tens of microns) and which produces on each track of the carrier a series of small magnetic areas which have magnetic inductions of opposed direction and of which the size is linked to the longitudinal recording density. For a magnetic disc, the number of magnetic areas per unit of length measured along the circumference of the track, is referred to as the longitudinal density. These areas, which are known as "elementary magnetic areas" are distributed throughout the length of the track.
Conversely, when the data of a given carrier travels before the gap of the associated transducer, the transducer may read the data by delivering electric playback or read signals via its electric input and/or output wires. These signals are in turn transmitted to electronic read circuits associated with the transducers.
The present trend in the development of magnetic transducers is to produce, in accordance with the technique for production of integrated circuits, transducers of ever smaller size (for example, having gaps of a size of the order of one to a few tens of microns). Transducers of this kind are manufactured, for example, by Compagnie Internationale Pour L'Informatique Cii-Honeywell Bull, and described in U.S. Pat. No. 3,723,665.
A transducer of this kind comprises a magnetic circuit formed by two pole pieces in thin layers, joined at one extremity in such a manner that they are coupled magnetically, and positioned with the other extremities close to the record carrier associated with the transducer in such a manner as to form a gap. The gap has a shape approximating that of a rectangle and its larger dimension is of the order of the radial width of the carrier tracks. The carrier extends substantially perpendicular to the pole pieces. One of the pole pieces is arranged on a substrate of insulating material. An electric winding is formed by thin conductive layers superposed in a direction at right angles to the plane of the pole pieces, and separated from each other by thin electrically insulating layers. The term "thin layer" denotes layers of which the thickness is of the order of a few Angstroms to a few microns. One part of the winding passes between the two pole pieces so that it may be said that these latter form a sheath for this part of the winding.
The magnetic record carrier associated with the transducer travels before the same perpendicular to the plane of the pole pieces and in a direction perpendicular to the larger dimension of the gap.
In current practice, the magnetic material forming the pole pieces is perferably anisotrophic and its axis of difficult magnetization is directed perpendicular to the record carrier.
Let us assme that each pole piece has a substantially rectangular shape. Its larger dimension is normal to the direction of travel of the record carrier. The axis of difficult magnetization of the magnetic material forming each pole piece is such that it has the same direction as the larger dimension of the pole piece.
It is known that the elementary magnetic areas forming the data of a track of the magnetic record carrier produce a magnetic leakage flux in direct proximity to the carrier surface. This magnetic flux is picked up by the gap of the integrated magnetic transducer associated with the same.
If .phi. denotes the magnetic flux penetrating into the gap of the transducer, and .phi..sub.m the magnetic flux passing through the winding of the same and consequently making it possible to generate a read signal, the transfer coefficient T of the integrated magnetic transducer is defined by the relationship T=.phi..sub.m /.phi..sub.t. For this transfer coefficient to be the maximum possible, one of the conditions which should be fulfilled is to have a high permeability of the thin magnetic layers forming the pole pieces, and this in the flux circulation direction. It is apparent that the utilization of an anisotrophic magnetic material whose axis of difficult magnetization extends perpendicular to the record carrier, allows the realization of this condition.
The advantages of an integrated magnetic transducer whose pole pieces are formed by an anisotrophic magnetic material, being the transducer described in the foregoing, are described in particular in the aforesaid U.S. Pat. No. 3,723,665. Improvements may be made in such transducers in the manner described in U.S. Pat. No. 4,016,601. These improvements make it possible to increase the efficiency of these transducers during writing as well as reading. Write efficiency is defined as the ratio between the magnetic flux generated by the transducer close to the gap, and the write current passing through the winding. Read efficiency is the ratio between the voltage available at the terminals of the winding and the magnetic flux which penetrates into the transducer at the level of the gap.
An integrated transducer as described in the aforesaid U.S. Pat. No. 4,016,601 comprises pole pieces which have a constriction at the level of the gap. The length of the constriction of the pole pieces in a plane parallel to the record carrier and situated in front of the carrier is equal to the large dimension of the gap, and is referred to as the "geometric track width LP.sub.g ". To form this constricvion, the pole pieces are machined by ionic attack over a depth of attack equal to p.sub.a (p.sub.a is of the order of a few microns) which produces concave lateral surfaces of the pole pieces referred to as S.sub.2 -S.sub.3, S'.sub.2 -S'.sub.3 situated, respectively, at either side of the gap. The surface of the pole pieces at the level of the gap, in the plane of the same, having a width LP.sub.g and parallel to the record carrier, is designated S.sub.1. If L is the width mf the pole pieces (small dimension of these pieces) prior to machining by ionic attack, L/LP.sub.g is commonly substantially equal to 2.
For reading, let us, for example, consider a particular track P of a magnetic disc having a supposedly insulated circular symmetry axis Ax.sub.p, and three different positions POS.sub.0, POS.sub.1, and POS.sub.2, which may be assumed by the transducer with respect to this track. In the position POS.sub.0, the gap of the transducer is centered perfectly in alignment with the track; this means that irrespective of the elementary magnetic area of the track envisaged, the axis Ax.sub.p of the track in this area and the symmetry axis Ax.sub.E of the gap normal to the plane of the pole pieces, are then situated in an identical plane normal to the carrier and are parallel to each other. In this position, the voltage of the read signal collected at the terminals of the transducer or winding is a maximum, or say A.
In the positions POS.sub.1 and POS.sub.2 which are symmetrical with respect to POS.sub.0 (the symmetry axis Ax.sub.E of the gap is shifted by an identical distance with respect to the axis Ax.sub.p), the voltage of the signal collected at the terminals of the winding is nil.
By definition, the read track width LP.sub.L refers to half the width of the space situated at either side of the position POS.sub.0 for which the voltage of the signal supplied by the winding is greater than or equal to 5% of A.
In current practice, it is observed that LP.sub.L is substantially greater than the write track width LP.sub.E ; as a matter of fact, another signal may be superimposed over the signal for reading the data of the track P, whether the track P is insulated or not. The following facts may enable a clearer grasp of the reasons for this action. During a first write operation on a track P of a record carrier by the integrated transducer associated with the same, the transducer, for example, occupies the position POS.sub.0 specified above. The circular symmetry axis of the track P then written is referred to as Ax.sub.p. If it is assumed that the track P is not insulated, P' and P" denote the tracks adjacent to the track P which are written on the carrier by the transducer. During a second write operation of an identical track P, the transducer occupies a position POS'.sub.0 very close to but different from POS.sub.0. The shift .delta. between these two positions is of the order of a few microns, in practice. This is attributable to the fact that the accuracy of the system for positioning the transducer with respect to the record carrier has a given maximum limit equal to the shift .delta., which cannot be improved. The new track P written consequently has a magnetic axis Ax'.sub.p staggered by .delta. with respect to Ax.sub.p.
Analogously, it is apparent that the two adjacent tracks P' and P" occupy a slightly different position on the carrier from that which they had occupied following the first write operation. Consequently, a zone containing magnetic data having the memory of the previous state of the carrier, that is of the state the carrier had following the first read operation for example, and whose width is substantially equal to the shift .delta., is thus present at either side of the new track P having the circular symmetry axis Ax'.sub.p.
The term "immediate surroundings of the track P" denotes the aggregate formed by the data of the zone having the width and by those of the two adjacent tracks P' and P".
The fact that the read track width LP.sub.L is greater than the write width LP.sub.E is essentially attributable to the following action: if, during the write operation the magnetic flux generated by the transducer allowing the writing of data on the carrier passes essentially through the surface S.sub.1 (a small part of this flux passes through the lateral concave surfaces S.sub.2 -S.sub.3 -S'.sub.2 -S'.sub.3), then in exchange, during the read operation, the concave lateral surfaces S.sub.2 and S.sub.3, S'.sub.2 and S'.sub.3 intercept a great part of the magnetic flux generated by the immediate magnetic surroundings of the track P. It is shown that the main reason for this action derives from the extensive anisotrophy of the material forming the pole pieces of the transducer.
The signal supplied by the winding of the transducer originating from the magnetic flux generated by the data of the track P, is referred to as "useful signal S", whereas the signal delivered by the winding originating from the magnetic flux generated by the immediate magnetic surroundings of the track P is referred to as "interference signal or noise signal B".
In current practice, the ratio S/B useful signal/noise of a transducer of this nature may be considered too low especially for some applications, particularly if the number of tracks of the carrier per unit of length is large. For a magnetic disc, this number is referred to as "radial density".
Solutions are known in practice for eliminating this drawback, the most current of which consists in forming an array of two magnetic transducers (integrated or not), having gaps of which the larger dimensions are parallel and different (it may thus be stated that the two gaps are parallel) and which have the same axis and the same symmetry plane perpendicular to their larger dimensions (and parallel to the direction of travel of the carrier).
The first gap, referred to as write gap, has a large dimension (geometric track width LP.sub.g) substantially equal to the width of the track written on the carrier LP.sub.E, whereas the second gap, referred to as the read gap, has a large dimension GD.sub.L which is smaller than LP.sub.g and which is such that the entire surface of the gap and of the pole pieces at the level of the same is situated wholly in alignment with the track which is in course of being read by the read transducer.
Nevertheless, such an array of two transducers requires high precision in its production in its production process, particularly in establishing coincidence between the axes of symmetry of the large dimensions of the two gaps. An array of this kind is difficult to produce and thus costly. It is demonstrated moreover that the electromagnetic efficiencies of each of the two transducers during the read as well as write operations are comparatively low.